Method for growing single crystal

ABSTRACT

A single crystal growing method for producing a high-quality and large-diameter single crystal of a compound semiconductor with a good yield, is disclosed. 
     A volatile element 2 is first put into a reservoir portion 1A of a quartz ampule 1. Further, a crucible 4 made of pBN, which contains a raw material 3A of a compound semiconductor, is placed in the quartz ampule 1, the vacuum sealing of which is then performed. While a vapor pressure controlling operation is performed, a furnace temperature distribution is controlled in such a manner that a vertical first temperature gradient α ° C./cm) in the vicinity of an outside wall of the quartz ampule corresponding to a raw melt 3B is smaller than a vertical second temperature gradient (β ° C./cm) in a range above the top end of the crucible 4 and simultaneously, the temperature is gradually lowered. Furthermore, α ranges from 51/D 2  to 102/D 2  ° C./cm, and preferably ranges from 58/D 2  to 83/D 2  ° C./cm (incidentally, the diameter of the single crystal is D cm). Additionally, β ranges from 1.06× to 1.72×° C./cm, more preferably, ranges from 1.19X to 1.46X ° C./cm ( incidentally, X is given by the following equation: X=√ Rρ/λnL), where the cooling rate of the furnace temperature and the coefficients of thermal conductivity, the specific gravity, the latent heat of melting and the formula weight of the crystal are assumed to be R ° C./hr, λ kcal/cm·hr·K, ρg/cm 3 , L kcal/mol and n g/mol, respectively).

TECHNICAL FIELD

The present invention generally relates to a method for growing a singlecrystal of a compound semiconductor and more particularly to a methodfor performing the melt growth of a single crystals of CdZnTe or CdTe inaccordance with a vertical gradient freezing (VGF) method whileperforming a vapor pressure control operation.

BACKGROUND ART

Hitherto, there have been known a vertical gradient freezing (VGF)method and a vertical Bridgman method as methods for growing singlecrystals of compound semiconductors. Further, there have been filedapplications concerning methods for controlling various growthrequirements for growing good-quality single crystals which have fewdislocations or the like.

For example, regarding VGF method, there has been well-known theinvention described in the Japanese Patent Application Publication(Examined) No. Tokuko-Hei 5-59873. The gist of this invention is asfollows. That is, a single crystal is grown downwardly from the centerof the surface of a raw melt by cooling the inside of a heating furnaceat a solid-liquid interface temperature gradient of 0.1 to 10° C./cm andat a cooling rate of 0.1 to 1° C./hr while the distribution oftemperature in the heating furnace is maintained in such a manner thatthe center of the surface of the raw melt is at the lowest temperatureand that as a temperature measuring position therein becomes radiallyoutwardly and downwardly farther away therefrom, the temperatureincreases.

Further, regarding Bridgman method, there have been filed applicationsconcerning a method for regulating a furnace wall temperature gradientalong the crystal and a furnace wall temperature gradient along the melton the basis of the coefficients of thermal conductivity of the crystaland the melt, respectively (as described in the Japanese PatentApplication Publication (Laid-Open) No. Tokukai-Hei 2-239181 and amethod for detecting temperatures at a plurality of points on the outercircumference of a crucible and for calculating and estimating thetemperature distribution in the crucible and the position and shape ofthe growth boundary face at regular time intervals according to thedetected temperatures, thereby controlling the growth requirements (asdescribed in the Japanese Patent Application Publication (Laid-Open) No.Tokukai-Hei 1-212291).

Moreover, there have been well-known methods of the inventions disclosedin the Japanese Patent Application Publication (Laid-Open) Nos.Tokukai-Hei 2-167882 and 3-183682 other than the previously filedaforementioned applications as a method for growing a crystal byestablishing a vertical temperature gradient by use of a vertical typeheating furnace similarly as in the cases of VGF method and Bridgmanmethod. In the case of the invention described in the Japanese PatentApplication Publication (Laid-Open) No. Tokukai-Hei 2-167882, a verticaltype vessel, on the top of which a nearly inverse-conical portion havinga small top opening is provided, is used. Further, the vessel is filledwith a raw melt. Moreover, a seed crystal is made to be in contact withthe raw melt through the small top opening. Thereby, the solidificationof the raw melt is performed downwardly. The invention described in theJapanese Patent Application Publication (Laid-Open) No. Tokukai-Hei3-183682 is an improvement of the invention described in the JapanesePatent Application Publication (Laid-Open) No. Tokukai-Hei 2-167882. Inthe case of the invention described in the Japanese Patent ApplicationPublication (Laid-Open) No. Tokukai-Hei 3-183682, a small bottom openingfor making excessive melt flow out therefrom is provided in the lowerportion of a vertical vessel similar to that used in the method of theinvention described in the Japanese Patent Application Publication(Laid-Open) No. Tokukai-Hei 2-167882. Thus, the failure or the like ofthe vessel due to the expansion of the volume thereof associated withthe solidification of the melt can be prevented by making the raw meltflow out from the small bottom opening.

Furthermore, regarding a method of using a lateral type heating furnacelike a horizontal Bridgman method and a horizontal gradient freezingmethod, there has been well-known a method of the invention described inthe Japanese Patent Application Publication (Examined) No. Tokuko-Sho53-5867. In the case of this method, a container containing a melt and aseed (i.e., a seed crystal) is charged into the lateral type furnace.Further, the solidification of the melt is started from the free surface(i.e., the top surface) thereof and is finished on the bottom surfacethereof by keeping the temperature on the surface of the melt, which isin contact with the bottom of the container, higher the temperature onthe top surface of the melt.

However, although a single crystal, the quality of which is higher thanas obtained by the vertical Bridgman method, can be obtained inaccordance with the control method described in the Japanese PatentApplication Publication (Examined) No. Tokuko-Hei 5-59873, all of thegrown crystals (ingots) are not always single crystals. Some of thegrown crystals are poly-crystallized. Thus, this method has a problem inthat the yield is not so high.

Additionally, in recent years, a good-quality and large-diameter singlecrystal, which has few crystal defects such as dislocations and is, forinstance, 4 inches in diameter, is desired. However, a method, by whichsuch a single crystal is produced with a good yield, has not beenestablished yet.

The present invention has been accomplished in view of suchcircumstances. An object of the present invention is to provide a methodfor growing a single crystal, by which good-quality and large-diametersingle crystals can be produced with a good yield.

DISCLOSURE OF THE INVENTION

In order to achieve the foregoing object, the inventor of the presentinvention and so on first put a material of CdZnTe into a pBN crucible(that is, a crucible made of pyrolytic boron nitride). Then, the vacuumsealing of such a crucible in a quartz ampule. Subsequently, a3-inch-diameter crystal of CdZnTe is grown according to VGF method.Further, the number of 25 mm×30 mm rectangular substrates obtained fromsuch a single crystal, which are compliant with product standards, isevaluated as an average value per growth (that is, the yield of singlecrystal substrates).

As a result, in contrast with the fact that the yield of the singlecrystal substrates is 4.6 to 4.8 sheets in the case of using a two-stagetype heating furnace 10 having two-stage heaters 11 and 12 asillustrated in FIG. 1(a) without controlling a vapor pressure(incidentally, a quartz ampule is filled with the simple substance of Cdof an amount corresponding to the content volume of the quartz ampuletogether with the raw material, the yield is increased to 10.9 sheets inthe case of using a quartz ampule with a reservoir portion and alsousing a three-stage type heating furnace 20 having three-stages ofheaters 21, 22 and 23 as illustrated in FIG. 1(b) while the vaporpressure is controlled in such a manner as to uniformly heat and holdthe reservoir portion containing the simple substance of Cd.

The inventor of the present invention and so on took note of thephenomenon that some of the three-stage heating furnaces, which are usedby controlling the vapor pressure in such a manner, provide good yieldsof the single crystal substrates but other three-stage heating furnacesprovide poor yields thereof. Further, the inventor of the presentinvention and so forth conducted investigations to determine the causeof the phenomenon. As a result, it was found that the furnaces providingthe good yields were different in the furnace temperature-distributioncharacteristics from those providing the poor yields. Further, theinventor of the present invention and so on come to find the properranges of a first vertical temperature gradient in the vicinity of theoutside wall of the quartz ampule and of a second vertical temperaturegradient in a range above the top end of the crucible from the furnacetemperature distributions of the three-stage type heating furnacesgiving good results.

Subsequently, the inventor of the present invention and so forth grew asingle crystal by using a six-stage type heating furnace provided withseven-stages of heaters 31, 32, 33, 34, 35, and 36, as illustrated inFIG. 1(c), so as to control the furnace temperature distribution in sucha manner that the first and second temperature gradients are within theproper ranges thereof, respectively. Consequently, the inventor of thepresent invention and so forth achieved a good result that the yield ofthe single crystal substrates is 21.6 sheets. Further, the yield of thesingle crystal substrates in the case of using a three-stage heatingfurnace could be increased to 20 sheets by improving the furnacetemperature distribution of the three-stage heating furnace 20 on thebasis of the furnace temperature distribution of the seven-stage typeheating furnace 30 and by inhibiting the use of furnaces which can notbe improved. Thus, the inventor of the present invention and so onconfirmed that it was important for increasing the yield of the singlecrystal substrates to put the first and second temperature gradientswithin the proper ranges thereof, respectively.

Moreover, the inventor of the present invention and so forth tried togrow a single crystal by increasing the diameter of the crystal, whichshould be grown, to a large diameter, that is, 4 inches. In this case,the inventor of the present invention and so forth gained the knowledgethat the first temperature gradient may have a further smaller value incomparison with the case where the diameter of a crystal is 3 inches.The method of the present invention has come to be completed by theformulation of the values of the first temperature gradient within theproper range thereof on the basis of the knowledge, which is performedby using the diameter of a single crystal as a parameter, and the heatcalculation to generalize the values of the second temperature gradientwithin the proper range thereof.

That is, in accordance with the present invention, there is provided amethod for growing a single crystal, which comprises the step of puttinga simple substance or a compound consisting of at least one kind of avolatile element, which composes a semiconductor compound, into areservoir portion of a quartz ampule provided therewith, the step ofplacing a crucible made of pBN, which contains a raw material of thesemiconductor compound, in the quartz ampule and then performing thevacuum sealing of the quartz ampule, the step of thereafter heating thequartz ampule in a heating furnace to thereby melt the raw material, thestep of heating the reservoir portion to a predetermined temperature andcontrolling the vapor pressure by applying the vapor pressure of thevolatile element, which has been put into the reservoir portion, to theinside of the quartz ampule, the step of controlling the furnacetemperature distribution of the heating furnace in such a manner thatthe vertical first temperature gradient (α ° C./cm) in the vicinity ofthe outside wall of the quartz ampule corresponding to the raw melt issmaller than the vertical second temperature gradient (β ° C./cm) in arange above the top end of the crucible (in the instant specification,this range indicates a range to a height of 10 cm above the top end ofthe crucible therefrom) and simultaneously gradually lowering thetemperature of the heating furnace so as to grow a single crystal of thecompound semiconductor downwardly from the surface of the raw melt insuch a manner that the first temperature gradient (α ° C./cm) rangesfrom 51/D² to 102/D² ° C./cm, more preferably, from 58/D² to 83/D² °C./cm where D denotes the diameter of the single crystal to be grown andon the other hand, the second temperature gradient (β ° C./cm) rangesfrom 1.06X to 1.72X ° C./cm, more preferably, from 1.19× to 1.46×° C./cmwhere X is given by the following equation: ##EQU1## in the case wherethe cooling rate of the furnace temperature and the coefficients ofthermal conductivity, the specific gravity, the latent heat of fusionand the formula weight of the crystal are assumed to be R ° C./hr, λkcal/cm·hr·K, ρ g/cm³, L kcal/mol and n g/mol, respectively.

Incidentally, there is a transition region between the first temperaturegradient (α ° C./cm) and the second temperature gradient (β ° C./cm).

Practically, as the aforementioned heating furnace, for example, aheating furnace which has: at least a first heating means, placed at aposition corresponding to the crucible, for controlling the firsttemperature gradient (α ° C./cm); a second heating means, placed abovethe first heating means, for controlling the second temperature gradient(β ° C./cm); a third heating means, placed under the first heatingmeans, for heating the reservoir portion; a fourth heating means and afifth heating means, which are placed above the second heating means andunder the third heating means and are used for restraining externalperturbations from affecting the first temperature gradient (α ° C./cm)and the second temperature gradient (β ° C./cm), respectively; and asixth heating means, placed between the first and third heating means,for restraining the first and third heating means from exerting aninfluence upon each other, is used. Further, the single crystal of thecompound semiconductor is that of, for instance, CdZnTe or CdTe.Preferably, the temperature of the reservoir portion is 770° to 830° C.More preferably, the temperature of the reservoir portion is 790° to820° C.

Here, the reason why the value of the first temperature gradient (α °C./cm) is within the aforementioned range is as follows. That is, in thecase of the method for growing a single crystal from the surface of themelt, a nucleus generated on the surface of the melt in the process oflowering the furnace temperature grows into a single crystal. If,however, the first temperature gradient (α ° C./cm) is too large, theconvection of the melt becomes strong. Thus the nucleus generated on thesurface of the melt is destroyed by the convection. Thereby, thegeneration of a nucleus becomes unstable. In contrast, if too small, theamount of heat supplied to the lower portion of the crucible becomessmall. Thus the temperature of the side wall of the crucible can not beheld as being higher or equal to the melting point. Further, in the casewhere the generation of a nucleus becomes unstable or in the case wherethe temperature of the side wall of the crucible becomes lower than themelting point, the poly-crystallization becomes very easy to occur. Thisis undesirable for growing a single crystal.

Further, the reason why the value of the second temperature gradient (β° C./cm) is within the aforementioned range is as follows. That is, ifthe second temperature gradient (β ° C./cm) is too small, the amount ofabsorbed heat becomes smaller than the amount of heat due to the latentheat of fusion. Thus the poly-crystallization occurs. In contrast, iftoo large, the temperature of the inside wall of the crucible at thelevel of the surface of the melt becomes equal to or lower than themelting point at an early stage of the growth of a crystal. Thence, thepoly-crystallization occurs.

Moreover, the reason why the temperature of the reservoir portion at thetime of growing a single crystal of CdZnTe or CdTe is within theaforementioned range is as follows. Namely, according to results ofexperiments made by the inventor of the present invention and so on, themaximum diameter of deposits in the grown crystal is 10 μm or so if thetemperature of the reservoir portion is within the range of 770° to 830°C. Further, the maximum diameter of deposits in the grown crystal isless than 5 μm if the temperature of the reservoir portion is within therange of 790° to 820° C. (see FIG. 4).

In accordance with the method of the present invention for growing asingle crystal, a single crystal is grown by controlling the furnacetemperature distribution of the heating furnace in such a manner thatα<β. Thus the amount of heat dissipated from the surface of the meltbecomes large, so that the growth of a single crystal starts from thesurface of the melt downwardly. At that time, the temperature of theinner circumference of the crucible is kept higher than or equal to themelting point of the raw melt during the growth of the single crystal.As a result, the single crystal having grown from the surface of themelt is floated on the melt. Thereby, the single crystal, the growth ofwhich has been advanced, is prevented from coming into contact with theinner wall of the crucible, on which a new nucleus may be generated anda poly-crystallization may occur. Consequently, large-diameter singlecrystals can be produced with good yields.

Further, a quartz ampule having a reservoir portion is used. Moreover,an operation of controlling a vapor pressure is performed by heating thereservoir portion, which contains a simple substance or a compoundcomprised of at least one kind of an easily volatilized element (i.e., avolatile element) among composing elements of a compound semiconductor,to a predetermined temperature and further, applying the vapor pressureof the volatile element to the reservoir portion. Thus, in comparisonwith the case where a predetermined quantity of a volatile element issimply put into a quartz ampule having no reservoir portion, thevariation in the applied vapor pressure is constrained. Moreover, thedecomposition of a surface portion of the grown single crystal, as wellas the vaporization of the raw melt, is effectively restrained. Thereby,lineage is reduced. Furthermore, the generation of a large deposit canbe prevented. Consequently, a good-quality single crystal can beobtained.

Moreover, the crucible made of pBN, the vacuum sealing of which isperformed in the quartz ampule, is used as a grower container. The sidewall of the crucible made of pBN has anisotropy in coefficient ofthermal conductivity, namely, the coefficient of thermal conductivity inthe vertical direction thereof is high, while the coefficient of thermalconductivity in the horizontal direction thereof is low. Thus the heatapplied to a lower portion of the crucible is easily transmitted to theupper portion of a side wall of the crucible. Furthermore, the amount ofheat radiated in the horizontal direction can be limited to a smallamount. This is extremely effective in keeping the temperature of theinner wall of the crucible higher than or equal to the melting pointduring the growth of single a crystal. Incidentally, needless to say,the crucible made of pBN has an advantage in preventing impurities frombeing contaminated. Moreover, a draft (i.e., a taper) for extracting acrystal, whose growth is finished, is provided in the crucible. That is,attention has been paid to the facilitation of extraction of a crystal.

Furthermore, if the diameter of a single crystal to be grown is assumedto be D cm, the first temperature gradient (α ° C./cm) in the verticaldirection in the vicinity of the outside wall of the quartz ampule,which corresponds to the raw melt, ranges from 51/D² to 102/D² ° C./cm.Thus the first temperature gradient (α ° C./cm) is not too large. Theconvection of the melt becomes not so strong that the nucleus generatedin the surface portion of the melt disappears. Consequently, thegeneration of the nucleus is stabilized. On the other hand, the firsttemperature gradient (α ° C./cm) is not too small. Further, anappropriate amount of heat is kept supplied to the lower part of thecrucible. Thus the side wall of the crucible is kept at temperaturesequal to or higher than the melting point during the growth of a singlecrystal. Consequently, large-diameter single crystals can be producedwith a good yield.

Especially, the controllability of the first temperature gradient (α °C./cm) and the second temperature gradient (β ° C./cm) is extremelyimproved by using the aforementioned multi-stage type furnace having atleast the first to sixth heating means as the heating furnace.Consequently, the yields of the single crystals are dramaticallyincreased.

Further, when growing a single crystal of CdZnTe or CdTe, the diametersof deposits in the grown crystal are small and 10 μm or so at themaximum if the temperature of the reservoir portion is within the rangeof 770° to 830° C. Furthermore, the diameters of deposits in the growncrystal are extremely small and the maximum diameter thereof is lessthan 5 μm if the temperature of the reservoir portion is within therange of 790° to 820° C. Consequently, an extremely excellent qualitysingle crystal can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(a), (b) and (c) are schematic sectional views for schematicallyillustrating the heating furnaces of three types used in VGF method.FIG. 2 is a schematic sectional view for illustrating a three-stage typeheating furnace used in this embodiment. FIG. 3 is atemperature-position characteristic diagram for showing the furnacetemperature distribution in a raw melt and a region above the raw meltof this embodiment. FIG. 4 is a characteristic diagram for illustratingthe correlation between the temperature of the reservoir portion of thisembodiment and the diameter of a deposit.

BEST MODE FOR CARRYING OUT THE INVENTION

In the case of a method of the present invention for growing a singlecrystal, while a vapor pressure controlling operation is performed byheating a reservoir portion of a quartz ampule to a predeterminedtemperature and keeping the reservoir portion at the predeterminedtemperature, the furnace temperature distribution of a heating furnaceis controlled in such a manner that the vertical first temperaturegradient (α ° C./cm) in the vicinity of the outside wall of the quartzampule corresponding to the raw melt is smaller than the vertical secondtemperature gradient (α ° C./cm) in a range above the top end of thecrucible. Thus a single crystal of the compound semiconductor is growndownwardly from the surface of a raw melt. Furthermore, the properranges of the first temperature gradient (α ° C./cm) and the secondtemperature gradient (β ° C./cm) are prescribed, respectively. First,the details of the prescribing of the proper range of each of the firstand second temperature gradients will be described hereinbelow. Thus itis based on this that the present invention is completed on the basis ofobjective facts and that the present invention is not subjected torestrictions due to the diameter and composition of a single crystal tobe grown and is, therefore, of universal application.

First, the inventor of the present invention and so on conducted anexperiment (i.e., Experiment 1) on the growth of an 80 mm (3-inch)diameter single crystal of CdZnTe by using a plurality of heatingfurnaces, the configuration of each of which was similar to that of thethree-stage type heating furnace 20 as illustrated in FIG. 1(b).Thereafter, the inventor of the present invention and so on made acomparison between the furnace temperature distribution characteristicsof a furnace having a good yield of single crystal substrates andanother furnace having a poor yield thereof. Further, as the result ofinvestigating the furnace temperature distribution regarding each of thecases where the grown crystal was a single crystal and where the growncrystal was poly-crystalline, it was revealed that when a single crystalwas obtained, the value of the first temperature gradient (α ° C./cm)ranged from 0.8 to 1.6° C./cm, more preferably, ranged from 0.9 to 1.3°C./cm and on the other hand, the value of the second temperaturegradient (β ° C./cm) ranged from 1.6 to 2.6° C./cm, more preferably,ranged 1.8 to 2.2° C./cm.

In the case of this Experiment 1, as illustrated in FIG. 2, the quartzampule 1 having the reservoir portion 1A was used. Further, the simplesubstance of a volatile element Cd2 was put into the reservoir portion1A. Moreover, the crucible 4 made of pBN, into which CdZnTe material 3Awas put, was placed in the quartz ampule 1. Thereafter, the vacuumsealing of the quartz ampule 1 was performed. Further, the raw material3A contained in the crucible 4 was melted by heating the crucible 4 byuse of the heater 22. Moreover, the vapor pressure was controlled byheating the reservoir portion 1A to a predetermined temperature in therange of, for example, 770° to 830° C. by means of the heater 23. Thelower part of the crucible 4 was heated by using the heater 21. Thetemperature in the heating furnace 20 was gradually lowered bycontrolling electric power to be supplied to the heaters 21, 22 and 23by use of a control unit (not shown) in such a manner that the desiredtemperature distribution occurred in the heating furnace 20.

Further, thermocouples 5A, 5B, 5C, 5D and 5E were provided in thevicinity of the outside wall of the quartz ampule 1. The temperature inthe vicinity of the outside wall of the quartz ampule 1 was measured.Moreover, the positions of the thermocouples 5A, 5B, 5C, 5D and 5Ecorresponded to the bottom of the quartz ampule 1, a position which wasat an altitude of 30 mm above the bottom thereof, a position which wasat an altitude of 60 mm above the bottom thereof, a position which wasat an altitude of 90 mm above the bottom thereof, and the cap 1B of thequartz ampule 1 (that is, a position which was at an altitude of 220 mmabove the bottom thereof), respectively. The values of the firsttemperature gradient (α ° C./cm) and the second temperature gradient (β° C./cm) were found on the basis of temperature measurement values ofthese five thermocouples 5A, 5B, 5C, 5D and 5E.

Meanwhile, the inventor of the present invention produced the six-stagetype heating furnace 30 having six stages of heaters 31, 32, 33, 34, 35,and 36 as illustrated in FIG. 1(c) by way of trial. Further, the furnacetemperature distribution in the six-stage type heating furnace 30 wasinvestigated by using a plurality of thermocouples similarly as in thecase of the three-stage type heating furnace 20 when growing a singlecrystal (Experiment 2). The temperature gradient between the bottom ofthe crucible and the initial position of the surface of the raw melt,that is, the first temperature gradient (α ° C./cm) was almost constantfrom the initiation of the growth to the termination thereof.Furthermore, it turned out that the precision of controlling thetemperature was improved in comparison with the three-stage type heatingfurnace 20.

In FIG. 1(c), the heater 31 is a first heating means which is placed ata position corresponding to the crucible and is operative to control thefirst temperature gradient. The heater 32 is a second heating meanswhich is operative to heat the space above the top end of the crucibleand to control the second temperature gradient. The heater 33 is a thirdheating means for heating the reservoir portion 1A. The heaters 34 and35 are fourth and fifth heating means for restraining externalperturbations from affecting the furnace temperature distribution. Theheaters 36 in sixth heating means for restraining the aforementionedheaters 31 and 33 from exerting an influence upon each other.Incidentally, the sixth heating means may be constituted only by oneheater. The number of stages of heaters is not limited to six. Thefurnace may have seven or more stages of heaters.

Subsequently, the inventor of the present invention and so on tried togrow a four-inch diameter single crystal of CdZnTe, similarly as in thecase of Experiment 1, by using the aforementioned six-stage type heatingfurnace 30 (Experiment 3). As a result, a single crystal was obtained oncondition that the first temperature gradient (α ° C./cm) was 0.53°C./cm and the second temperature gradient (α ° C./cm) was 1.83° C./cm.This revealed that the proper value of the first temperature gradient (α° C./cm) became small when the diameter of a crystal to be grown becamelarge. The inventor of the present invention and so on obtained theknowledge that the first temperature gradient (α ° C./cm) should be ininverse proportion to the section of the crystal (that is, the square ofthe diameter thereof). In the case of this experiment, the vaporpressure was controlled by heating the reservoir portion 1A to thepredetermined temperature in the range of, for example, 770° to 830° C.

As the result of considering the permissible range of variation oftemperature due to various causes on the basis of the aforementionedknowledge, it was found that the appropriate range of the firsttemperature gradient (α ° C./cm) was 51/D² to 102/D² ° C./cm, morepreferably, from 58/D² to 83/D² ° C./cm where D designates the diameterof a single crystal to be grown.

Incidentally, FIG. 3 is a temperature-position characteristic diagramfor showing an example of the furnace temperature distribution when afour-inch diameter single crystal was obtained. Additionally, in FIG. 3,each plot indicates a temperature measured by the thermocouple.

Next, the inventor of the present invention and so forth made thefollowing calculation by establishing a precondition so as to estimate atemperature gradient on a solid-liquid interface, which was obtainedfrom the temperatures in a single crystal, a raw material and the innerwall of the crucible but could not be measured during the growth of acrystal. The precondition was that an amount Q1 of heat transmitted in asingle crystal grown and solidified was equal to the latent heat Q2 offusion of the raw melt (i.e., Q1=Q2). When making the calculation, thefollowing numerical values and symbols were used.

The cooling rate of the furnace temperature: 0.1K/hr.

The diameter of the single crystal: 100 mm (4 inches).

The coefficient of thermal conductivity of CdTe (incidentally, a valuethereof in the proximity of the melting point): 1.5 W/m·K=1.29kcal/m·hr·K

The specific gravity of CdTe: 5.86 g/mol.

The latent heat of fusion of CdTe: 12 kcal/mol.

The temperature gradient on the solid-liquid interface: X k/cm.

Thus, the amount Q1 of heat transmitted in a single crystal grown andsolidified was given by the following equation: ##EQU2##

Further, the latent heat Q2 of fusion of the raw melt was obtained bythe following equation: ##EQU3##

Furthermore, Q1=Q2. Therefore,

    1.013 X =2.301/X

Finally, the value of X is obtained as follows by solving this equation:

    X=1.51 K/cm.

That is, the temperature gradient on the solid-liquid interface isestimated as 1.51 K/cm.

It was seen from the foregoing description that even if the temperaturegradient (α ° C./cm) measured in the neighborhood of the outside wall ofthe quartz ampule was less than 1.0 K/cm similarly as in the case of thegrowth of, for instance, the aforementioned four-inch diameter singlecrystal of CdZnTe, the practical temperature gradient in the practicalsingle crystal and on the solid-liquid interface was more than such avalue and was a little smaller than the temperature gradient (1.83 K/cm)in the space above the top end of the crucible. Therefore, as will bedescribed later, the generalization of the second temperature gradient(β ° C./cm) could be achieved on the basis of the precondition that theamount Q1 of heat transmitted in a single crystal grown and solidifiedwas equal to the latent heat Q2 of fusion of the raw melt.

Further, the actual temperature gradient in the single crystal and thesolid-liquid interface was 1.51 K/cm. Thus the growing rate of thesingle crystal was estimated as 0.66 mm/hr.

    0.1 (K/hr)+1.51 (K/cm)=0.66 (mm/hr)

Consequently, the substantial growth time of a 60 mm length singlecrystal was obtained as 91 hours from the following expression forcomputation:

    60 (mm)÷0.66 (mm/hr)=91 (hr).

Further, usually, the temperature at a position on the outside wall ofthe quartz ampule, which corresponds to the surface of the raw melt atthe time of starting the growth, is 1117° C. The growth of a singlecrystal is completely finished at the time when the temperature of thecrystal ampule reaches the melting point (1098° C.) of the raw meltafter cooling the outside wall from such a temperature of the outsidewall at a cooling rate of 0.1 K/hr. Here, if the difference between thetemperatures of the crucible inner safe and the outside wall of thequartz ample was estimated as 5K (which was a value obtained by apreliminary experiment conducted by putting the thermocouples into theraw liquid), the growth of the single crystal came to be alreadycompleted when the temperature of the outside wall of the quartz ampulewas 1103° C. Thus, because the substantial growth time was 91 hoursamong 140 hours during which the temperature changed from 1117° C. to1103° C., a growing nucleus occurs in the surface portion of the rawmelt when the temperature of the quartz ampule when the temperature ofthe quartz ampule became equal to 1112° C. after the lapse of a time of49 hours posterior to the starting of the growth. Thereby, it was foundthat a single crystal was obtained by accurately controlling the furnacetemperature distribution until tens or hundreds hours were lapsed afterthe growth was started.

Next, the inventor of the present invention and so on performed thegeneralization of the second temperature gradient (β ° C./cm) on thebasis of the results of the heat calculation conducted in theaforementioned practical example in order to realize the application ofthe present invention to the growth of a single crystal of othercompound semiconductors such as InP, GaAs, ZnSe and ZnTe. A preconditionat that time was the same as the precondition that the amount Q1 of heattransmitted in a single crystal grown and solidified was equal to thelatent heat Q2 of fusion of the raw melt (i.e., Q1=Q2). Incidentally,the numerical values, such as the cooling rate of the furnacetemperature, used for the calculation of the precondition weresymbolized as follows:

The cooling rate of the furnace temperature (K/hr): R.

The section of a single crystal (cm²): S.

The coefficient of thermal conductivity of the crystal (kcal/cm·hr·K):λ.

The specific gravity of the crystal (g/cm³): ρ.

The latent heat of fusion of the crystal (kcal/mol): L.

The temperature gradient on the solid-liquid interface (K/cm): X.

The formula weight of the crystal (g/mol): n.

Thus the amount Q1 of heat transmitted in a single crystal grown andsolidified was given by the following equation:

    Q1=S×X×λ

Moreover, the latent heat Q2 of fusion of the raw melt was obtained bythe following equation:

    Q2=R÷X×S×ρ÷n×L

Since Q1=Q2,

    S×X×Xλ=R÷X×S×ρ÷n×L

Therefore, by solving this equation for X, the following equation wasobtained: ##EQU4##

As the result of considering the permissible range of variation oftemperature due to various causes on the basis of this, it was foundthat the appropriate range of the second temperature gradient (β °C./cm) was 1.06X to 1.72X ° C./cm, more preferably, 1.19× to 1.46×°C./cm.

Then, the inventor of the present invention and so on made singlecrystals of CdZnTe grow a plurality of times (Experiment 4), similarlyas in the case of Experiment 1, by setting the first temperaturegradient (α ° C./cm) and the second temperature gradient (β ° C./cm) asvalues selected from those of the generalized proper range in order toverify the validity of the generalization of the first temperaturegradient (α ° C./cm) and the second temperature gradient (β ° C./cm),which was performed in the aforementioned manner. The surfaceorientation of the obtained single crystal is always the orientationcorresponding to a crystal plane (111), which is a preferential growingorientation. Further, an etch-pit density (EPD) was measured by etchinga substrate cut out of the single crystal by use of HF etchant. Themeasured value of EPD ranged from 4×10⁴ to 6×10⁴ cm⁻². This valuecorresponds to a dislocation density (see K. Nakagawa et al., Appl.Phys. Lett., 34 (1979) 574). Moreover, the half-value width of raysdiffracted by a crystal plane (333), which was measured by performingfour-crystal X-ray diffraction measurement, was equal to or less than 20seconds (i.e., 7 to 20 seconds). It was found from the results of themeasurement that the crystallinity of the obtained single crystal ofCdZnTe was good. Thus the validity of the present invention wasconfirmed.

Subsequently, the inventor of the present invention and so on made anexperiment (i.e., Experiment 5) so as to prescribe the most appropriaterange of the temperature of the reservoir portion 1A serving as a factorin determining the vapor pressure of the volatile element, which was anobject of the vapor pressure controlling operation. The results of thisexperiment are illustrated in FIG. 4. As is inferred from this figure,if the temperature of the reservoir portion 1A was 770° to 830° C., thatis, the vapor pressure of Cd was nearly 1.0 to 2.0 atm, the deposit inthe grown crystal was small and was 10 μm or so in diameter at themaximum. Further, if the temperature of the reservoir portion 1A was790° to 820° C., that is, the vapor pressure of Cd was nearly 1.3 to 1.8atm, the deposit in the grown crystal was extremely small and was 5 μmor so in diameter at the maximum. Thus, it was found that thetemperature of the reservoir portion 1A was preferably 770° to 830° C.,and was more preferably 790° to 820° C. Incidentally, when thetemperature of the reservoir portion 1A was almost 813° C. (815° to 825°C. or so) and the vapor pressure of Cd was 1.7 atm, n-type and p-typeinversion or conductive regions occurred in the crystal. Further, thedeposits were smallest in size.

In the case of Experiment 5, the reservoir portion 1A was heated to750°, 780° C., almost 813° and 850° C. Further, in the case of each ofthese temperatures, a single crystal of CdTe was made to grow, similarlyas in the case of Experiment 1. Furthermore, the sizes of the depositsin the obtained crystal were measured by using an infrared microscope.

Thus, in accordance with the present invention, not only in the case ofusing the aforementioned seven-stage type heating furnace as the heatingfurnace, but also in the case of using the aforementioned three-stagetype heating furnace, better-quality and large-diameter single crystalscan be produced with a good yield.

Incidentally, the present invention is not subjected to restrictions dueto the aforementioned embodiment thereof. For example, the presentinvention may be applied to the growth of single crystals of othercompound semiconductors such as InP, GaAs, ZnSe and ZnTe other than CdTeand CdZnTe. Further, the diameter of the single crystal to be grown isnot limited to 3 inches and 4 inches. Moreover, the number of stages ofheaters of a heating furnace used at that time is not limited to threeand seven.

Furthermore, instead of performing the growth of single crystals bymeasuring the temperature in the vicinity of the outside wall of thequartz ampule 1 at all times by means of the thermocouples 5A, 5B, 5C,5D and 5E provided therein similarly as in the case of theaforementioned embodiment, a single crystal may be grown by controllingthe first temperature gradient (α ° C./cm) and the second temperaturegradient (β ° C./cm) while measuring temperature at a point formeasuring temperature, which is provided at a place other than thevicinity of the outside wall of the quartz ampule 1, by utilizing therelation between the temperatures at the newly provided temperaturemeasuring point and the vicinity of the outside wall of the quartzampule 1, which has been preliminarily obtained by performing apreliminary experiment.

In accordance with the method of the present invention for growing asingle crystal, the vertical second temperature gradient (β ° C./cm) ina range above the top end of the crucible is larger than the verticalfirst temperature gradient (α ° C./cm) in the vicinity of the outsidewall of the quartz ampule corresponding to the raw melt. Thus, thesingle crystal having grown from the surface of the melt is floated onthe melt in such a manner as not to come into contact with the innerwall of the crucible. Thereby, a new nucleus can be prevented from beinggenerated on the inner wall of the crucible. Consequently, it becomeseasy to obtain single crystals.

Further, an operation of controlling a vapor pressure is performed byheating the reservoir portion, which contains an easily volatilizedelement. Thus, the variation in the vapor pressure is constrained.Further, the vapor pressure can be kept at an appropriate level.Moreover, the decomposition of a surface portion of the grown singlecrystal, as well as the vaporization of the raw melt, is moreeffectively restrained. Thereby, lineage is reduced. Furthermore, thegeneration of a large deposit can be prevented. Consequently, thequality of a single crystal can be improved.

Moreover, the crucible made of pBN has anisotropy in coefficient ofthermal conductivity. Thus the heat applied to a lower portion of thecrucible is easily transmitted to the upper portion of a side wall ofthe crucible. Furthermore, the amount of heat radiated in the horizontaldirection can be limited to a small amount. This is extremely effectivein keeping the temperature of the inner wall of the crucible higher thanor equal to the melting point during the growth of single a crystal.

Furthermore, if the first temperature gradient (α ° C./cm) in thevertical direction in the vicinity of the outside wall of the quartzampule, which corresponds to the raw melt, ranges from 51/D² to 102/D² °C./cm, more preferably, from 58/D² to 83/D² ° C./cm, the loss of thenucleus due to the convection of the melt can be prevented.Consequently, the generation of the nucleus is stabilized. Moreover, anappropriate amount of heat is kept supplied to the lower part of thecrucible. Thus the side wall of the crucible is kept at temperaturesequal to or higher than the melting point during the growth of a singlecrystal. Consequently, it becomes easy to obtain single crystals.

Especially, the controllability of the first temperature gradient (α °C./cm) and the second temperature gradient (β ° C./cm) is extremelyimproved by using the multistage type furnace having at least a firstheating means for controlling the first temperature gradient (α °C./cm), a second heating means for controlling the second temperaturegradient (β ° C./cm), a third heating means for heating the reservoirportion, a fourth heating means and a fifth heating means used forrestraining from being affected by external perturbations and a sixthheating means for restraining the first and third heating means fromexerting an influence upon each other. Consequently, the yields of thesingle crystals are dramatically increased.

Further, when growing a single crystal of CdZnTe or CdTe, the diametersof deposits in the grown crystal are small and 10 μm or so at themaximum if the temperature of the reservoir portion is within the rangeof 770° to 830° C. Furthermore, the diameters of deposits in the growncrystal are extremely small and the maximum diameter thereof is lessthan 5 μm if the temperature of the reservoir portion is within therange of 790° to 820° C. Consequently, an extremely excellent qualitysingle crystal can be obtained.

As stated above, the present invention has an advantage in that afurther-excellent-quality and large-diameter single crystal can beproduced with a good yield thereof.

Industrial Applicability

As described above, the present invention is most effective forproducing a single crystal of a compound semiconductor, especially, forproducing a single crystal of a compound semiconductor in accordancewith VGF method by controlling a vapor pressure.

We claim:
 1. A method for growing a single crystal, comprising the stepsof:putting a semiconductor compound having at least one volatile elementinto a reservoir portion of a quartz ampule provided therewith; placinga crucible made of pBN, which contains a raw material of thesemiconductor compound, in the quartz ampule and vacuum sealing thequartz ampule; heating the quartz ampule in a heating furnace to therebymelt the raw material; heating the reservoir portion to a predeterminedtemperature to vaporize the volatile element of the semiconductorcompound in the reservoir portion thereby creating a vapor pressure andcontrolling the vapor pressure by applying the vapor pressure of thevolatile element in the reservoir portion, to the inside of the quartzampule; controlling a temperature distribution of the heating furnacesuch that a vertical first temperature gradient in a vicinity of theoutside wall of the quartz ampule corresponding to the raw material issmaller than a vertical second temperature gradient in a vicinity abovethe top end of the crucible, and simultaneously gradually lowering thetemperature of the heating furnace so as to grow a single crystal of thesemiconductor compound downwardly from the surface of the raw materialsuch that the first temperature gradient ranges from 51/D² to 102/D² °C./cm, where D denotes the diameter of the single crystal to be grown,and the second temperature gradient ranges from 1.06X to 1.72X ° C./cm,where X is given by the following equation: ##EQU5## wherein Rrepresents a cooling rate of the furnace temperature, λ represents acoefficient of thermal conductivity, ρ represents a specific gravity, Lrepresents a latent heat of fusion and n represents a formula weight ofthe crystal.
 2. The method for growing a single crystal according toclaim 1, wherein said heating furnace has: at least a first heatingmeans, placed at a position corresponding to the crucible, forcontrolling the first temperature gradient; a second heating means,placed above the first heating means, for controlling the secondtemperature gradient; a third heating means, placed under the firstheating means, for heating the reservoir portion; a fourth heating meansand a fifth heating means, which are placed above the second heatingmeans and under the third heating means and are used for restrainingexternal perturbations from affecting the first temperature gradient andthe second temperature gradient, respectively; and a sixth heatingmeans, placed between the first and third heating means, for restrainingthe first and third heating means from exerting an influence upon eachother.
 3. The method for growing a single crystal according to claim 1,wherein the single crystal of the compound semiconductor is a singlecrystal of CdZnTe or CdTe.
 4. The method for growing a single crystalaccording to claim 3, wherein the temperature of the reservoir portionis within the range of 770° to 830° C.
 5. The method for growing asingle crystal according to claim 2, wherein the single crystal of thecompound semiconductor is a single crystal of CdZnTe or CdTe.
 6. Themethod for growing a single crystal according to claim 5, wherein thetemperature of the reservoir portion is preferably within the range of770°0 to 830° C.